Defect chemistry of the BIMEVOXes

Since their discovery in 1988, the BIMEVOXes have been the subject of significant research due to their high oxide ion conductivity at relatively low temperatures. The development of these materials is briefly reviewed. The defect structure of the BIMEVOXes is discussed and used to construct general defect equations for solid solution formation. Two limiting models are proposed by which solid solution formation can occur. In the Equatorial Vacancy (EV) model, vacancies are located exclusively in bridging sites in the vanadate layer. In contrast, the Apical Vacancy (AV) model assumes vacancies are located exclusively in non-bridging apical sites in the vanadate layer. The general defect equations can be used to predict theoretical solid solution limits for all types of substitutions for vanadium in Bi4V2O11-delta. These limits are found to vary not only with the charge of the dopant ion, but also with the coordination number of the metal dopant. In most cases it is found that the EV model yields theoretical solid solution limits close to those observed. The EV model is also used to present a mechanism for ionic conduction in BIMEVOXes, which involves movement of equatorial oxide ions/vacancies between vanadium octahedra and tetrahedra with the formation of a five-coordinate vanadium intermediate.